This invention generally relates to printing apparatus for photosensitive media and more particularly relates to a printing apparatus and method for recording images onto a high contrast photosensitive medium.
Over many years, apparatus and techniques for high-volume printing of images onto photosensitive media have been continuously refined and improved to reduce cost, boost efficiency, and maximize speed. In particular, printing of motion picture print films, distributed to theaters and other exhibitors, has benefited from this ongoing improvement process, so that today""s methods for providing print films are considered by many film printing labs to be efficient and cost-effective.
It is illustrative to give a brief overview of the workflow for conventional motion picture print film manufacture, as shown in FIG. 1. The image input to the printing process at a contact printer 120 is a master negative film 122, carefully printed and prepared by a film lab as an intermediate for the printing process. Unimaged print film 124 is the material input to this process. At high rates of speed, a contact printing method is used at a printhead 126 to expose images from film master negative 122 onto unimaged print film 124, to produce a positive print on an exposed print film 124xe2x80x2. Exposed print film 124xe2x80x2 is then developed, dried, and packaged for distribution to theaters and other exhibitors. In the print process itself, several hundred exposed print films 124 xe2x80x2 can be made from a single master negative film 122.
With the advent of digital imaging capabilities, it is recognized that there are opportunities for improvement in methods and apparatus used for motion picture print film manufacture. In particular, two-dimensional spatial light modulator arrays, such as liquid crystal device (LCD) arrays and digital micromirror device (DMD) arrays, have been proposed for use in various printing applications. Just a few examples of printing apparatus and methods using these devices are disclosed in U.S. Pat. No. 6,215,547 (Ramanujan et al.) which discloses the use of a reflective LCD with a 3-color LED source; U.S. Pat. No. 6,480,259 (Wong et al.) which discloses a printing apparatus using polarization modulation with added polarization components for increasing contrast ratio; and U.S. Pat. No. 6,163,363 (Nelson et al.) which discloses the use of a DMD array for providing a printing apparatus with optimized contrast ratio.
In spite of the capabilities and advantages offered by digital imaging technologies, a number of significant hurdles remain. Among the more significant obstacles is writing speed. Two-dimensional spatial light modulators do not yet provide sufficient light levels or exhibit sufficient refresh rates for use in high-speed print film preparation, making it impractical to substitute these devices in printing applications where high-speed film exposure is currently used. This is particularly important since, as can be readily appreciated, high speed apparatus used for printing onto print film represent considerable capital investment. Until digital imaging technologies can offer significant advances in speed and lower cost, conventional contact printing techniques will likely be employed for the vast bulk of print film preparation.
However, while digital technologies may still not be optimal for print film exposure, spatial light modulators are being employed in print applications for preparing the negative master, where printing speeds are not critical and where digital imaging offers other advantages. For example, U.S. Pat. No. 6,215,547 (Ramanujan et al.) teaches the use of an LCD spatial light modulator for printing an image onto photosensitive media. Using approaches such as that disclosed in U.S. Pat. No. 6,215,547, it would be possible to use LCD spatial light modulators for writing images onto an intermediate negative film. Conventional contact-printing exposure techniques would then be used for printing from this intermediate master negative to the print film.
It is well recognized in the imaging arts that some amount of image degradation is inevitable with each intermediate stage. Thus, for example, even when printing from a negative of the highest quality, with a contact printer having ideal exposure levels and timing, some loss of image quality at the print film is inevitable. Typically, for example, there is some small amount of motion between the negative and the print in a contact printer, resulting in some loss of sharpness. Additionally, stray light in the printing process can contribute to some loss of contrast. Thus, it follows that, even with the relatively good performance of high-quality contact printers, there are advantages for image quality in eliminating an intermediate stage. Writing directly to print film, without the use of an intermediate master negative, would have inherent value for improved image quality. Even though it may take more time to write directly to print film, as compared with imaging by contact printing, there can be compelling reasons for providing print films having exceptionally high quality. For example, select motion picture theaters could charge a premium for showing a first run film having very high image quality, in comparison with most other theaters that project from print film prepared in the conventional manner.
Thus, it can be seen that there is a perceived need for being able to print directly to print film, using digital imaging techniques, even if this process is more time consuming than the conventional high-volume process. One alternative is to provide a printing apparatus expressly for this purpose, able to accept print film and to image directly onto the print film, without any intermediate negative stage. However, this first alternative would likely prove too costly for commercial use. Another alternative would be to begin with an existing printer designed for digital printing of master negative film media, and adapt such a printer to the additional task of imaging directly onto print film. A printer of this type could then be operated either to print a master negative or to print onto print film.
Adapting a printer for writing directly to print film presents a number of significant challenges, however. A first challenge is due to differences in media response. Referring to FIG. 2, there is shown a density versus log exposure curve for a typical intermediate negative film medium, such as would be conventionally used for master negative film 122 in FIG. 1. Referring to FIG. 3, on the same scale as FIG. 2, there is shown a density versus log exposure curve for a typical print film, such as would be conventionally used as a contact print film 124 in FIG. 1. Comparing FIGS. 2 and 3, it can be appreciated that there is significant difference in response between negative film and print film. The negative film can be characterized as a low contrast film. Print film, meanwhile, is characterized as a high contrast film. The slope of the D log E curve shows the relative gamma, or contrast characteristic, of the film medium. In terms of contrast, the negative film, as shown in FIG. 2, exhibits much lower contrast than the print film, as shown in FIG. 3. For example, a typical intermediate negative film has a gamma of about 1.0. By comparison, a typical print film has a gamma of nearly 5.0 at the point of steepest slope.
The photosensitive media of FIGS. 2 and 3 differ significantly in terms of dynamic range. Briefly, dynamic range for a photosensitive medium is based on the difference between the brightest and dimmest regions of exposure. The dynamic range of the light exposure required for the negative medium represented in FIG. 2 is relatively high, on the order of 708:1. That is, the brightest exposure must be about 700 times greater than the dimmest exposure in order to produce a negative with a density range from zero to slightly over 2.0. For the print medium of FIG. 3, however, the dynamic range of required light exposure is relatively low, at approximately 89:1. The steeper slope (gamma) of the D log E curve for the print medium of FIG. 3 means that a much smaller ratio of exposure light levels is needed to span the density range from minimum to maximum.
Because of the difference in dynamic range for the two photosensitive media of FIGS. 2 and 3, it would not be suitable to simply substitute one medium for the other in a printing apparatus. For example, with mere substitution of the media of FIG. 3 into a writer set up for the media of FIG. 2, only a small range of digital values spans the difference in density from 0 density to maximum density (Dmax). Since a small difference in code value and log exposure value results in a large difference in density, image contouring effects, therefore, are much more likely with the high contrast media of FIG. 3.
Labeling of the x-axis in FIGS. 2 and 3 shows how code value would map to density if a typical printer calibration look-up table (LUT) were created. In FIG. 2, the code values are spread across a significant extent of the log exposure region, where the curve is essentially linear. Using a conventionally generated calibration LUT, FIG. 3 shows how input code values spread over the same exposure range would map to output densities on the print film. In FIG. 3, the lower code values and some of the upper code values are mapped to flat areas of the curve, so that large changes in code value in these ranges produce little change in density. With the response of FIG. 3, code values in these flat areas are effectively lost, leaving only the remaining code values to cover the entire dynamic range of the film medium. A printing apparatus handling this type of media must be configured for a reduced exposure range, re-mapping code values in the input image data to code values for the reduced exposure range of the imaging medium. That is, code values must be redistributed over the useful density range of the film. If code values are not suitably redistributed, it is possible to cause contouring artifacts in the image. Contouring occurs when a single increment in code value causes a corresponding density change that exceeds the threshold of visual perception. Visible density contours in an image produce an unwanted effect analogous to contour lines on a weather map.
Conventional negative/positive film systems purposely employ high and low contrast films. A low contrast film (having a gamma of approximately 0.6-0.7) is used to capture the original scene. By utilizing a low contrast film for this function, a wide scene dynamic range is recorded. Also, a low contrast film is more tolerant of small exposure errors, allowing correction to be made during the printing stages without severely altering overall tone reproduction. Intermediate film, with a gamma close to 1.0 is used to produce intermediate positive and negative copies of the camera negative. The gamma of 1.0 prevents contrast build up during the numerous duplication stages of motion picture production. Print film, then, is a high contrast film. The high contrast is necessary to produce an overall tone reproduction that is pleasing when viewed in a darkened theater. The full system contrast is the product of the gamma of all of the stages of production. The system contrast is slightly greater than 1.0 to compensate for the eye""s diminished contrast perception when viewing a scene in a darkened room. Developed over many decades, this elegant system works remarkably well. It is particularly instructive to observe that this conventional method employs a combination of low- and high-contrast media, where desirable characteristics of both low- and high-contrast media are utilized at different points in the workflow and are harmonized in order to provide a pleasing visual output. It is not surprising that interposing digital printing into this conventional workflow is not xe2x80x9cseamlessxe2x80x9d and can introduce problems with tone reproduction and image artifacts.
The difference in contrast illustrated in FIGS. 2 and 3 means that print film media accentuates artifacts when compared with negative media. For example, print film tends to accentuate LCD image artifacts such as non-uniformity and defective or marginal pixels, which may not be as readily apparent on the negative film medium alone. In the case of the films of FIGS. 2 and 3, print film can accentuate perceptible artifacts by a factor as high as 4 to 5. Therefore, there is a need to compensate for high contrast response when a printing apparatus is used for direct printing to a print film.
It is known in the imaging arts to use a look-up table (LUT) to compensate for non-linearities introduced by a photosensitive medium or by light modulation components such as acousto-optic modulators or liquid crystal devices. The LUT provides a flexible mechanism, allowing modification for media or modulation device differences and is often used for calibration purposes.
One method for calibrating a printing apparatus to the response characteristics of a photosensitive medium is by adjusting the mapping of digital data, creating a LUT to correlate input data values to output exposure levels, such as is described in commonly-assigned U.S. patent application Ser. No. 10/000,967, filed Nov. 2, 2001, entitled xe2x80x9cCalibration Method for a Printing Apparatus Using Photosensitive Mediaxe2x80x9d by James Erwin and William Miller. Using the method of U.S. patent application Ser. No. 10/000,967, input code values are mapped to device code values by correlating exposure energy values to density throughout the range of possible input data values, at a given bias voltage level. While this method is useful for calibration, particularly where the resolution of a light modulator has higher resolution than the input data, U.S. patent application Ser. No. 10/000,967 does not address uniformity problems that can be perceptible when changing between low- and high-contrast media. For this reason, in addition to data value mapping using LUTs, contrast response characteristics of photosensitive print film may require additional adjustment of printing hardware.
Thus, it can be seen that there is a need for a printing apparatus and suitable method for adapting a printing apparatus to photosensitive media having different contrast response characteristics.
It is an object of the present invention to provide a printing apparatus and method for adapting a printing apparatus suited to printing onto a plurality of types of photosensitive medium, differentiated by contrast response. With this object in mind, the present invention provides a printing apparatus for selectively printing an image from image data onto any member of a set of photosensitive media, wherein the set comprises at least a first photosensitive medium and a second photosensitive medium, wherein the first photosensitive medium has a relatively low contrast response with respect to the second photosensitive medium, the apparatus comprising:
(a) a spatial light modulator for forming an image by modulating a polarization state of an incident light according to the image data, the spatial light modulator having at least a first set of setup voltage conditions for printing onto the first photosensitive medium and a second set of setup voltage conditions for printing onto the second photosensitive medium;
(b) an adjustable polarizing component in the path of output light from the spatial light modulator, the adjustable polarizing component having at least a first contrast setting for the first photosensitive medium and a second contrast setting for the second photosensitive medium; and
(c) a control logic processor for selectively setting at least the first set of setup voltage conditions for the first photosensitive medium and the second set of setup voltage conditions for the second photosensitive medium.
From another aspect, the present invention provides a method for printing an image from image data onto a photosensitive medium having a predetermined contrast response, comprising:
(a) setting voltage conditions for a spatial light modulator, based on the contrast response of the photosensitive medium;
(b) adjusting polarization components to reduce the contrast of imaging optics according to the contrast response of the photosensitive medium;
(c) directing an incident light having a predetermined polarization state toward a spatial light modulator;
(d) modulating the incident light according to the image data in order to form a modulated image-bearing beam; and
(e) directing the modulated image-bearing beam toward the photosensitive medium.
A feature of the present invention is the use of adjustable polarization components for adjusting contrast in a printing apparatus.
It is an advantage of the present invention that it allows a single printer to be readily adapted to print onto different photosensitive media having a range of contrast response characteristics. Using the method of the present invention, a printer could be adjusted to accommodate two or more different media types, each media type having specific setup parameters. In a particular case, the same printer used for imaging onto a negative film medium could be used to image directly onto a positive film medium, effectively eliminating a step in film reproduction processing.
It is a further advantage of the present invention that it allows adjustment for providing improved uniformity on a high contrast medium.